Certain types of power converters use resonant processes to efficiently transfer energy from an input source to a load. One example of such a power converter, called a Sine-Amplitude Converter (“SAC”), is described in detail in Factorized Power Architecture with Point of Load Sine Amplitude Converters, Vinciarelli, U.S. Pat. No. 6,984,965 (assigned to VLT, Inc. of Sunnyvale, Calif., the entire disclosure of which is incorporated herein by reference) (the “SAC Patent”). A block diagram of a half-bridge sine amplitude converter 10 is shown in FIG. 1. The SAC comprises SAC power conversion circuitry 100 (shown connected to power source 50 and load 60) and a SAC controller 20 which controls the turning ON and OFF of switches within the power conversion circuitry 100. A simplified and idealized summary of the operation of the sine amplitude converter 10 is as follows (a detailed description of operation may be found in the SAC Patent): switches S1 110 and S3 130 are closed when the voltages across switches S1 110 and S3 130, and the resonant portion of the primary current Ipri, are each substantially zero, initiating a power transfer interval. Closing switch S1 110 causes an essentially sinusoidal flow of resonant current Ipri in the primary winding at a characteristic resonant frequency, FR, and period, TR, defined by a resonant circuit comprising a resonant inductance Ls 150 and a resonant capacitance Cres 160. When the sinusoidal current flow completes a half-cycle, and the current Ipri returns substantially to zero, switches S1 110 and S3 130 are opened. An energy recycling interval following the opening of switches S1 110 and S3 130, allows the transformer 80 magnetizing current to charge and discharge circuit parasitic capacitances such that the voltages across switches S2 120 and S4 140 decline toward zero. When the voltage across switches S2 120 and S4 140 are at their minimum, i.e. substantially zero (or at a relatively low value, dependent upon the magnitude of the magnetizing energy), they are turned ON to initiate another power transfer interval. Each converter operating cycle comprises two power transfer intervals of equal length and two energy recycling intervals of equal length; one half-cycle of the converter operating cycle comprises a single power transfer interval and an associated energy recycling interval. Each power transfer interval is substantially equal to one-half of the characteristic period, TR/2; each operating cycle, of duration T, is therefore greater than or equal to TR, depending on the length of the energy recycling interval. The converter operating frequency, Fop, is defined as the inverse of the length of the converter operating cycle: Fop=1/T. Typical SACs may have converter operating frequencies between 1 MHz and 4 MHZ, or higher.
Conventionally, control of a SAC requires that circuit conditions be monitored in order to determine the proper times at which to turn switches ON and OFF. As shown in FIG. 1, the SAC controller 20 may comprise End-of-Cycle Sense circuitry 30 to receive information from the SAC power conversion circuitry 100 to establish when switches should be turned ON and OFF. For example, the voltage across one or more of the switches may be monitored to establish the timing of a zero-voltage switching (“ZVS”) or zero-current switching (“ZCS”) event, or the current flowing in the transformer 80 may be monitored to establish the timing of a ZCS event. The monitoring circuitry may provide a feedback signal to the End-of-Cycle Sense circuitry 30 in the controller 20 which responds to the feedback signal by altering the states of one or more switches.
Contemporary high-frequency power converters typically use MOS-gated power switching devices, such as MOSFETs and IGBTs, which have an essentially capacitive gate control terminal. Efficiently recycling the energy during the turning ON and OFF of such a device may increase overall converter operating efficiency. One way to efficiently recycle the energy is to use a resonant technique. Apparatus and methods for resonant recycling of capacitive gate energy are described in: High Efficiency Floating Gate Driver Circuit Using Leaking Inductance Transformer, Vinciarelli, U.S. Pat. No. 6,107,860; Lossless Gate Driver Circuit for a High Frequency Converter, Steigerwald, U.S. Pat. No. 5,010,261; High Frequency Control of a Semiconductor Switch, Toile et al, U.S. Pat. No. 7,602,229; and Gate Driving Circuit, Inoshita, U.S. Pat. No. 7,091,753.